In a 2016 NFL playoff game between the Green Bay Packers and the Arizona Cardinals, the game went to overtime. On the overtime coin flip, the referee threw the coin in the air and it didn't flip. The Packers, who had lost the toss, convinced the referee to flip the coin again (with no change in the outcome). If the result had changed, I imagine this would have caused considerable controversy. Is there actually a rule in the NFL rulebook that the coin must flip over at least once for the toss to be valid?
As a Packer fan I remember the situation and recall that at the time this happened there was no rule demanding the coin to actually flip. Here is a video of Dean Blandino VP of officiating at the time explaining the situation, that by rule the coin didn't actually have to flip, but common sense dictated that it should in order for the flip to be fair. And so Clete Blakeman the referee did the right thing to redo the coin toss.
Fun fact, after the game Aaron Rodgers mentioned he wanted to switch to heads for the second toss, but the referee did the second flip before he had the chance to choose a coin side again. After the 2015-2016 season, the NFL updated its rules to explicitly handle such situation. Section 2, Article 2 (coin toss) states:
If the coin does not turn over in the air or the toss is compromised in any way, the Referee shall toss it again. The captain’s original call may not be changed.
Well, it also turned out that they heard what Rodgers said after the game. The new rule also states that the captain's choice stands, thus in case of a re-flip, the teams cannot change the side of coin picked before the first toss.